Your wife's stress is her problem to solve. It's admirable that you want to help but you're part of the problem in her eyes, not part of the solution. There's nothing wrong with asking if there is anything you can do to help but if she declines, you've offered and it remains her problem to solve.
Me: 31, W: 29 T: 4 M: 2 Kids: 3 (SS: 7, SD: 4, D: 3) Separated, still living together: Nov 2013 Separate bedrooms: Feb 2014 W working away; kids with me: Nov 2014